What factor contributes significantly to background radiation from terrestrial sources?

Uranium in the ground is a primary contributor to background radiation from terrestrial sources. The earth's crust contains trace amounts of uranium, which is a naturally occurring radioactive element. As uranium undergoes radioactive decay, it produces radon gas and other decay products, which can emit radiation and contribute to the background radiation levels we experience.

Unlike other options, such as X-ray machines or nuclear waste, which are associated with human-made sources of radiation, uranium is a natural component of the earth’s geology. Additionally, smoking, while harmful and linked to various health risks, is not a significant source of background radiation; it primarily introduces exposure to chemical toxins rather than radiological exposure. Therefore, the presence of uranium in the ground plays a vital role in the natural background radiation that we encounter daily.